1. Field of the Invention
The present invention relates generally to echo canceller systems in communication networks. More particularly, the present invention relates to methods and systems for detection of infinite and finite echo return loss (ERL).
2. Background Art
Subscribers use speech quality as the benchmark for assessing the overall quality of a telephone network. A key technology to provide a high quality speech is echo cancellation. Echo canceller performance in a telephone network, either a TDM or packet telephony network, has a substantial impact on the overall voice quality. An effective removal of hybrid and acoustic echo inherent in telephone networks is a key to maintaining and improving perceived voice quality during a call.
Echoes occur in telephone networks due to impedance mismatches of network elements and acoustical coupling within telephone handsets. Hybrid echo is the primary source of echo generated from the public-switched telephone network (PSTN). As shown in FIG. 1, hybrid echo 110 is created by a hybrid, which converts a four-wire physical interface into a two-wire physical interface. The hybrid reflects electrical energy back to the speaker from the four-wire physical interface. Acoustic echo, on the other hand, is generated by analog and digital telephones, with the degree of echo related to the type and quality of such telephones. As shown in FIG. 1, acoustic echo 120 is created by a voice coupling between the earpiece and microphone in the telephones, where sound from the speaker is picked by the microphone, for example, by bouncing off the walls, windows, and the like. The result of this reflection is the creation of multi-path echo, which would be heard by the speaker unless eliminated.
As shown in FIG. 1, in modern telephone networks, echo canceller 140 is typically positioned between hybrid 130 and network 150. Generally speaking, echo cancellation process involves two steps. First, as the call is set up, echo canceller 140 employs a digital adaptive filter to adapt to the far-end signal and create a model based on the far-end signal before passing through hybrid 130. After the near-end signal including the echo signal, passes through hybrid 130, echo canceller 140 subtracts the far-end model from the near-end signal to cancel hybrid echo and generate an error signal. Although this echo cancellation process removes a substantial amount of the echo, non-linear components of the echo may still remain. To cancel non-linear components of the echo, the second step of the echo cancellation process utilizes a non-linear processor (NLP) to eliminate the remaining or residual echo by attenuating the signal below the noise floor.
However, digital connections do not include a hybrid that converts a four-wire physical interface into a two-wire physical interface, rather in digital connections a four-wire physical interface is converted into another four-wire physical interface and, thus, digital connections do not cause impedance mismatches that give rise to an echo. For this reason, digital connections are referred to as zero echo path or infinite ERL. When infinite ERL is detected, the correct model of echo is zero, i.e. the near-end signal should pass through without any subtraction of echo signal from the near-end signal. If existence of a digital connection or infinite ERL is not properly and timely detected by an echo cancellation system, the adaptive filter of the echo cancellation system may train on noise from the near-end or a leakage of the near-end talker signal, which can leak due to untimely detection of double talk. As a result, the adaptive filter generates an echo model based on the near-end signal and not the far-end signal. As described above, the echo cancellation system subtracts the echo model from the near-end signal, which does not include any echo, since there is an infinite ERL. As a result, the echo cancellation system becomes an echo generator, rather than an echo canceller.
Furthermore, if an infinite ERL is not timely and properly detected, the NLP may falsely engage, and cause signal choppiness in the near-end signal. For this reason, the infinite ERL must be properly and timely detected, such that the NLP is bypassed.
During a given connection, the echo path may also change, and a digital connection may be changed to a connection having a hybrid, which converts a four-wire physical interface into a two-wire physical interface and, thus, giving rise to an echo signal. Untimely detection of a finite ERL will, of course, prevent the adaptive filter to generate an echo model for subtraction from the near-end signal, and echo signals will not be cancelled and will be heard by the far-end talker.
Accordingly, there is a need in the art for timely and proper detection of infinite ERL and finite ERL to timely disable and enable echo cancellation, respectively.